In an ignition coil, high voltage current is generated using an igniter to spark an ignition plug. A conventional ignition coil shown in FIG. 5 has a resinous cylindrical case 80 that forms an outer shell of the ignition coil. The resinous cylindrical case 80 receives a center core 82, a secondary spool 83, a primary spool 86 and an outer core 89 that are coaxially arranged. A secondary coil 84 is wound on the secondary spool 83. A primary coil 87 is wound on the primary spool 86. The outer core 89 is located on the circumferentially outer side of the case 80.
A present engine valve structure 115 shown in FIG. 7 has a water jacket 117 that is jumboized to enhance cooling capacity of the ignition plug, which is inserted and secured to a plughole 119, to improve fuel efficiency and engine power. In this structure, the plughole 119 needs to be small sized. Specifically, the inner diameter of the plug hole 119 is conventionally set between 23 mm and 25 mm, however, the inner diameter of a plug hole 119 is presently demanded to be less than 20 mm.
In a conventional ignition coil according to JP-A-2003-232272, a cylindrical case is provided on the circumferentially outer side of a primary coil. Specifically, in the conventional ignition coil shown in FIG. 5, the cylindrical case 80, which is arranged on the circumferentially outer side of the primary coil 87, has large wall thickness that is between 0.6 mm and 1.2 mm. Therefore, it is difficult to reduce the outer diameter of the ignition coil, i.e., the inner diameter of the plughole to be less than 22 mm in this conventional structure.
As shown in FIG. 6, in a conventional ignition coil according to JP-A-9-199353, a cushion sheet 112 is arranged on the radially inner side with respect to an outer core 109. Specifically, a center core 100, which partially forms a closed magnetic passage, a primary coil 103, a secondary coil 106 and the outer core 109 are arranged from the center to the outer circumferential periphery. The cushion sheet 112 is made of an aramid polymer paper, and is inserted into the inner circumferential space of the outer circumferential core 109.
Thickness of the cushion sheet 112 is not specifically described in JP-A-9-199353, however, the cushion sheet 112 needs sufficient thickness to obtain sufficient shock-absorbing property. The thickness of the cushion sheet 112 may be an obstacle to reduction of the diameter of the ignition coil.